One significant market for paints is in the repainting of motor vehicles after crash damage repair or refurbishment. This is often referred to as Vehicle Refinish. One of the most common types of vehicle refinish paints currently on the market comprises a hydroxyl functional polymer and a polyisocyanate crosslinker in an organic solvent. Because the polymer and the crosslinker react together from the time they are mixed, they are generally supplied to the end user as two separate components which are mixed together shortly before use. For this reason, they are often called “two-pack” paints.
In practice, the component containing the polymer also contains any components such as pigments, fillers, extenders or other additives which may be required to give the paint its final colour or to modify its properties. Pigments or extenders are particulate materials which are held in stable dispersion in the solventborne composition by a pigment dispersant. In making the compositions, the pigments, fillers and extenders are usually firstly made into a pigment millbase. A pigment millbase is made by mixing the pigments, fillers and extenders and the pigment dispersant, usually with a little solvent, in a high shear mixer such as a ball mill so as to thoroughly separate the pigment particles and coat them with dispersant. The resulting millbase is then mixed with polymer and solvent to form the composition known as the “paint pack” as sold to the end user. The user then mixes this paint pack with a solution of the crosslinker in organic solvent, referred to as the “hardener”, just prior to use.
There is environmental and legislative pressure to reduce the emission of organic solvents from coating compositions when they are applied and dried to form a paint film. One way to achieve this is to use waterborne compositions.
Producing a waterborne equivalent of the vehicle refinish two pack paint described above is not simply a question of replacing all of the solvent with water. Each component must be modified in order to make it water soluble or dispersible. In particular, the modification would necessarily involve the introduction of hydrophilic groups into the crosslinker which must be made water dispersible. The introduction of hydrophilic groups into the crosslinker has been found to make the final paint film more sensitive to water, rendering this route unattractive in producing high performance refinish paints with good water resistance.
In an alternative (see EP-358979) the polymer is produced in an organic solvent. On completion of the polymerisation reaction the organic solvent is removed and the polymer and is made into an aqueous solution or dispersion. Pigment may then be added to the aqueous polymer solution or dispersion (if required) before addition of the isocyanate crosslinkers.
It is however desired to obtain coatings having improved water resistance compared to the water resistance of coatings produced in accordance with EP-358979. This is achieved in accordance with the present invention by the use of a solution in an organic solvent of polymer having functional group and hydrophilic groups having dispersed therein a waterborne pigment dispersion.
One proposed alternative method of producing a waterborne two pack composition uses a solventborne paint pack. The polymer and the crosslinker are both chosen to be soluble in an organic solvent and the pigment millbase is made using a pigment dispersant compatible with the solvent. The polymer is dissolved and the pigment is dispersed in the organic solvent to form the solventborne paint pack. The end user dissolves the crosslinker in the solventborne paint pack to form an activated paint pack and then disperses this in water to form the waterborne coating composition ready for use.
It has been found that this route can provide waterborne two pack compositions with equivalent performance to solventborne compositions. The problem with this route is that although the majority of the carrier fluid can be water, there is still a relatively high level of solvent present from the solventborne paint pack. Levels of solvent in essentially waterborne compositions are often expressed as volatile organic content (voc) in grams per liter of composition excluding water (g/l). For primer composition, these types of composition still have a voc in the region of 350 g/l. This is less than conventional solventborne primer compositions but existing and forthcoming regulations are aiming at 250 g/l, and a still lower level is clearly desirable. For single layer topcoat compositions these types of composition still have a voc in the region of 420 g/l, which again is less than conventional solventborne topcoat compositions but forthcoming regulations are aiming at 340 g/l, and a still lower level is clearly desirable.
We have now discovered that it is possible to reduce the voc of these types of composition by the use of a waterborne pigment millbase. This waterborne millbase is dispersed in a solution of the polymer in an organic solvent so as to form the paint pack. The polymer has hydrophilic groups so that it acts to stabilise the water-in-oil emulsion of the millbase in the polymer solution. It also has functional groups which will react with the crosslinker. A crosslinker is used which is not hydrophilically modified and which is soluble in the organic solvent. Just prior to use, the crosslinker is added to this dispersion and dissolved in the solvent and then this mixture is finally dispersed in water. This oil-in-water dispersion is also stabilised by the polymer. This method allows crosslinker without hydrophilic groups to be used, while at the same time allowing compositions to be made having a volatile organic content significantly lower than those made by conventional routes.